September 29, 2019November 7, 2022 by gary_mail@hotmail.com

Standard electrode potentials

The electrochemical series described in a basic level article is, in fact, a tabulation of the half-cell standard electrode potentials, E^o, of various chemical species. Just as the metal reactivity series compares the reactivity of metals, the electrochemical series lists the reactivity of metals and ions in an electrochemical cell. A summary of the electrochemical series is as follows:-

The more positive the potential of a half-cell is, the more spontaneous its reaction is from left to right. Hence, when we connect two half-cells,

the overall cell potential is:

$E_{cell}=E_{right}-E_{left}=E_{red,cathode}-E_{red,anode}$

For example, the combined redox reaction of Zn²⁺/Zn and Cu²⁺/Cu is

$E_{right}\; or\; E_{red,cathode}:H_2(g)+Cu^{2+}(aq)\rightleftharpoons 2H^+(aq)+Cu(s)$

$E_{left}\; or\; E_{red,anode}:Zn(s)+2H^+(aq)\rightleftharpoons Zn^{2+}(aq)+H_2(g)$

Adding the two cell reactions, we have the E_cell reaction equation, whose E_cellvalue is E_right– E_left:

$Zn(s)+Cu^{2+}(aq)\rightleftharpoons Zn^{2+}(aq)+Cu(s)$

$E_{cell}=0.34\; V-(-0.76\; V)=+1.10\; V$

The above diagram can be simplified by omitting the standard hydrogen electrodes to give:

Question

What If we place the Cu²⁺/Cu half-cell on the left and the Zn²⁺/Zn half-cell on the right?

Answer

We will have

$E_{cell}=-0.76\; V-0.34\; V=-1.10\; V$

This implies that the overall electrochemical cell reaction is not spontaneous, which is not true.

next article: Analysing reactions using standard electrode potentials

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